Sunken Mirror

Hi, where can I find the sunken mirror room? I click the mirror in the Temple of Nodens and the only thing it says is "A strange creature looks back at you from the mirror". Please help

OMG I went and started a new game and talked to Russian I realized he gives a Sphere of Illumination so the Mirror is passable and I don't know where I put it, I know its unsellable but I checked my house and everything. Is there any way to get another? or do I have to restart all over again, or is the Sunken Mirror just a side quest? I am worrying because I think the Sunken Mirror Room is a crucial part of the Main Mission. Also my last saves also did not have the sphere

Edit: OMG ITS SELLABLE!
Is there a shop where I can buy another one?

Half or the Innkeeper sells it but you might have to camp out outside his inn until,it appears in his inventory.

Half or the Innkeeper sells it but you might have to camp out outside his inn until,it appears in his inventory.

Thank you for replying but, where is "half" or the "innkeeper" located because I tried camping in Albion Village's inn for about an hour, but no luck at all, only thing she sells are wood, candles, fireproof chest.

Sorry, that was a typo. Halbor the Innkeeper sells it. He's in Lydney Village.

Vanora the Innkeeper in Albion Village sells it too. But it's randonmness is one so, as mentioned, you have to sleep and keep checking the inn's inventory. Errin the innkeeper in Avalonia should sell it as well.